Poxviruses have evolved multiple mechanisms to modulate and evade host immunological responses. The orthopoxvirus, vaccinia is currently being employed as a vaccine to induce protective immunity and prevent transmission of the class A bioterrorism agent, smallpox (variola). Immunization with vaccinia can induce both cellular and humoral immunity in humans, yet concerns have been raised with respect to vacccine efficiency, longevity and safety particularly with regards to immunocompromised individuals. Elucidating novel mechanisms of poxviral immune evasion, is therefore an important priority with regards to developing improved, safer vaccines. This proposal will test the hypothesis that vaccinia virus disrupts MHC class II-restricted antigen presentation, thus compromising the activation of host cellular immune responses during infection. Studies in humans indicate that months to years after immunization with vaccinia, virus-specific CD8+ and CD4+ T cells can be detected. Yet, an early, transient decrease in T cell responsiveness has been reported and linked to potential defects in antigen presentation. Pilot studies here demonstrated that vaccinia infection of professional antigen presenting cells disrupts MHC class II-restricted antigen presentation to T cells. Aim 1 of this proposal, will test whether viral inhibition of class II presentation is linked to the structure, abundance, or compartmentalization of an antigen. Aim 2 will determine whether vaccinia-derived proteins directly block the interaction of MHC class II molecules with T cell receptors, as well as investigating viral mechanisms to thwart antigen processing. Aim 3 will examine vaccinia viral infection in vivo, specifically monitoring early effects of the virus on class II presentation pathways within distinct antigen presenting cells. The overall goal of this work is to elucidate novel mechanisms by which vaccinia and other orthopoxviruses subvert MHC class II antigen processing and presentation. The results obtained should prove useful in the design of new interventions and improved vaccines to overcome the immunomodulatory properties of poxviruses and thus, enhance protective immunity. Class II presentation is an essential function of professional antigen presenting cells. Investigations here in project 4 are therefore, integral to the program's goal to define and understand mechanisms by which poxviruses subvert antigen presenting cell function.